This invention relates to a vertical sync counter for a television receiver and more particularly to a vertical sync counter having a first mode of operation wherein the counter counts up to a first preselected number of horizontal pulses and a second mode of operation where the counter counts up to a second preselected number of horizontal pulses to enable the TV receiver to be readily fabricated or manufactured for use in TV systems having a different number of lines per picture.
Some TV receivers include both a free running vertical oscillator and a free running horizontal oscillator. However, for reasons of economy and manufacturing simplicity, some TV receivers have the vertical oscillator replaced with a counter that is driven by the output of the free running horizontal oscillator. This is possible because of the fixed relationship between the vertical and horizontal frequencies. The number of lines in a TV picture vary throughout the world. For example, in the United States each TV picture contains five hundred twenty-five lines whereas in Europe each TV picture contains six hundred twenty-five lines. It is desirable to be able to manufacture TV receivers that are operable in both systems without the necessity of using two different vertical counters or two different manufacturing techniques and/or systems. This is particularly true when the TV receiver is fabricated on one or more integrated circuits.
Accordingly, one object of this invention is to provide an improved vertical sync counter for a TV receiver.
Another object of this invention is to provide a vertical sync counter having a first mode of operation wherein the counter is reset automatically after counting up to a first predetermined number of pulses and a second mode of operation wherein the counter is automatically reset after counting up to a second different number of predetermined pulses.
Still another object of this invention is to provide a vertical sync counter which is readily enabled to operate in one of two modes of operation to enable a TV receiver to be easily manufactured for use in TV systems having different numbers of lines per picture.